Conventional transmission systems for broadcasting have each a backup transmitter for backup, in many cases, in addition to a transmitter to be used for actual broadcasting, in order to secure the redundancy of a transmission apparatus. As one of the simplest systems, there is an active/backup system which has one backup transmitter for one transmitter. In this system, the number of required backup transmitters is equal to the number of transmitters. For this reason, in the case of a broadcast station having a plurality of transmitters, a lot of backup transmitters are required, and thereby the burden of capital investment thereof becomes large. As one means to suppress the burden of capital investment and secure a backup transmitter, there is an (N+1) system which has one backup transmitter for a plurality of transmitters.
At the time of switching to a backup transmitter, switching is performed so that an input signal is inputted to the backup transmitter, and an output signal from the backup transmitter is outputted. In an (N+1) system, since the backup transmitter becomes a backup for not less than two transmitters, change in a frequency and an output level and so on is required, at the time of switching. At this time, since the frequency is previously known, it is possible to previously adjust the backup transmitter in accordance with the frequency.
In addition, it is possible to previously adjust the output level similarly, but the output level may be changed during operation. When the output level is changed, at the time of switching to the backup transmitter, it becomes necessary to adjust the output level in accordance with the change.
In addition, in a digital transmitter, in order to compensate nonlinear distortion of the amplifier, there is a case to perform control for compensation of the nonlinear distortion, by feeding back an output signal of the amplifier. In this case, when the output level fluctuates, it is necessary to perform adjustment again in accordance with the fluctuation.
As described above, in the conventional art, when adjustment of an output level of a transmitter is performed during operation, adjustment of the output level is required at the time of switching to a backup transmitter.
In addition, when nonlinear distortion compensation of a feedback system is performed, it is necessary to perform switching after the setting in accordance with adjustment of a feedback level and the changed nonlinear distortion characteristic is performed. For this reason, when adjustment of an output level of a transmitter is performed during operation, there was a problem that a switching time is excessively required.